1. Field of the Invention
This invention relates to recovery of oil from a hydrocarbon bearing layer and more specifically to use of radiofrequency ground heating to extract oil from a hydrocarbon bearing layerin-situ.
2. Description of Related Art
Oil shale, contains no oil and little extractable bitumen, but does contain organic matter composed mainly of an insoluble solid material called kerogen. Shale oil can be generated from kerogen during pyrolysis, a treatment that consists of heating the oil shale to elevated temperatures (typically, greater than 350.degree. C.). The amount of worldwide potential oil reserves from kerogen in oil shale is estimated to be about 4.4 trillion barrels according to B. P. Tissot and D. H. Welte in Petroleum Formation and Occurrence: A New Approach to Oil and Gas Exploration, Springer-Verlag, New York, 1978, p. 235. Of this, approximately 2/3, or 2.9 trillion barrels, are contained in the United States in the Green River Shales of Colorado, Utah and Wyoming. The next largest oil shale reserves are the Irati Shales of Brazil, with about 1.1 trillion barrels, while other large quantities of oil shale are found in Australia, Canada, China, Estonia, France, Great Britain, Spain, Sweden, Switzerland, Uruguay, Yugoslavia and Zaire.
Because of the large supply in the United States, a practical method of extracting this oil at competitive prices (less than 20 per barrel) could substantially change the energy balance between the United States and the rest of the world.
Below an oil yield of 6 gallons/ton, more energy is expended in heating the oil shale to pyrolysis than the calorific value of the kerogen contained within it. This is defined as the lower production limit for commercial oil shales. The average oil shale richness in the Green River Shales is about 20 gallons/ton.
Bridges and Taflove of the Illinois Institute of Technology Research Institute (IITRI) proposed mining a shaft through material above oil shale, known as overburden, to the top of the oil shale and inserting an array of electrodes into the oil shale starting from this shaft. This method for RF heating of oil shale is described in U.S. Pat. No. 4,144,935, Apparatus and Method For In-situ Heat Processing of Hydrocarbonaceous Formations by J. Bridges and A. Taflove issued Mar. 20, 1979. Their electrode array is designed to be a "triplate," where the center electrode row is at high potential and the adjacent rows on either side at ground potential. The IITRI process is extremely expensive in the United States because the Green River shale typically has an overburden of 600-800 feet. Any underground mining operation to install an electrode array at this depth is uneconomic at today's oil prices.
A somewhat different method of RF shale heating utilizes an array of specially designed dipole antennas inserted into the ground, described in U.S. Pat. No. 4,140,179, In-situ Radio Frequency Selective Heating Process by R. S. Kasevich, M. Kolker and A. S. Dwyer issued Feb. 20, 1979. A problem with this approach is that the antenna elements must be matched to the electrical conditions of the surrounding formation. As the formation is heated, the electrical conditions can change, and the dipole antenna elements have to be removed and changed, which presents significant practical and economic difficulties.
Other prior art methods of extracting oil from oil shale involve the use of linear resistive heating elements embedded in the oil shale. These linear resistive heating elements apply heat to the oil shale immediately adjacent the elements. The heat distribution to the remainder of the oil shale is controlled by the rather slow thermal diffusivity of the oil shale. One such method is disclosed in U.S. Pat. No. 4,886,118 Conductively Heating a Subterranean Oil Shale to Create Permeability and Subsequently Produce Oil by Peter Van Meurs, Eric de Rouffignac, Harold Vinegar and Michael Lucid issued Dec. 12, 1989 ("7-spot thermal conductivity patent"). This invention employs a seven-spot pattern to apply heat to the oil shale through thermal conduction. Each repeating pattern has six resistive heating wells surrounding an oil production well. The resistive heating elements heat oil shale bounded by the heating wells to pyrolysis. Oil is collected by the production wells and is pumped to the surface. The main disadvantage of thermal conduction heating is that thermal conduction sources have to be very close together. For example, this invention employs 50-foot spacing between the heating elements. Because of the low heat conductivities of oil shale, the maximum heat injection rate per well for thermal conduction wells is about 200 watts/foot, so that thermal conduction heating requires on the order of 15-20 injectors per acre. This density of heating wells can be very expensive and renders the process not economically feasible at today's oil prices.
At present, there is a need for a method of extracting oil from a hydrocarbon bearing layer, such as oil shale, that is economical and efficient.